To ensure high availability and operational safety of the electrical power supply and to guarantee personnel safety in the operator area of electrical installations, power supply networks are increasingly employed whose active components are separated from the earth potential. In this kind of power supply network, called IT system, an active conductor can present an insulation fault without the running operation of the installation having to be interrupted because no closed circuit can form in this first fault case owing to the ideally infinitely high impedance value between the conductor and earth. In this context, an insulation fault is understood to be a faulty state of the IT system which leads to a drop in the insulation resistance below the acceptable level of insulation. It becomes clear from this consideration that the resistance against earth in the network to be monitored (insulation resistance; also fault resistance in case of a fault) must be monitored constantly because a possible further fault in another active conductor (second fault) would cause a fault loop and the flowing fault current in connection with an overcurrent protection device would lead to a shutdown of the installation and to operational downtime. Actively measuring insulation monitoring devices known from the state of the art are connected between the network conductors and earth in the main branch of the IT system and superpose a measuring voltage on the network which leads to a current flow proportional to the insulation resistance. This measuring current causes a voltage drop at the measuring resistance of the insulation monitoring device, which is evaluated and results in a warning if a limit value, which can be preset, is exceeded.
In conjunction with continuous insulation monitoring, a fast and reliably executable localization and clearance of the insulation fault (insulation fault locating) is indispensible.
If an insulation fault occurs in an IT system, it is detected and reported by an insulation monitoring device. Upon this report, insulation fault location begins in that a test signal generator generates a test signal (test current) and, according to the state of the art, feeds it into the power supply system at a central point in the main branch. The test current flows through the live lines, the fault resistance and the earth lead back to the feeding point of the test current. This test current signal can be registered by all measuring current transformers in that circuit.
The objective in using an insulation fault locating system in branched IT systems is the most sensitive detection possible of insulation faults present in the subsystems and the identification of the subsystem with the largest insulation fault, i.e. the smallest insulation fault resistance. Since the test signal generator is virtually always designed to be current-limited, the test current is shared between all insulation fault resistances and network leakage capacitances present in the IT system. This means that the residual current flowing in a subnetwork is dependent not only on the size of the fault resistance in said subnetwork but also on other fault resistances and network leakage capacitances present in the IT system. The respective residual current measuring device in the subnetwork can safely detect insulation faults in the subnetwork to be monitored only starting from a minimum differential current determined by the resolution and the measuring error of the measuring system. Thus, the sensitivity of the insulation fault locating system is determined by the overall configuration of the IT system and can be negatively influenced.
In known designs of insulation fault locating systems, the test signal is additionally dependent on the nominal voltage of the IT system to be monitored. This, too, is another parameter that influences the sensitivity of the insulation fault locating system and increases the complexity of locating the insulation fault. Another particularity arises in insulation fault locating systems that feed pulse-shaped test signals, mainly in the form of a square-wave signal sequence, into the IT system. In this case, the transient effects due to scattered network leakage capacitances must be considered. The ratio of the network leakage capacitances upstream and downstream of the differential current measuring device in the subnetwork additionally determines the achievable sensitivity of the insulation fault locating device.
These deliberations show that the efficient use of an insulation fault locating system requires knowledge and consideration of the electrical parameters of the entire IT system. Knowledge of the cross-connection impedances, i.e. of the complex-value impedances carrying leakage current between the subsystems, would be advantageous.
The implementation of the insulation fault location, in particular in extensive, highly branched IT systems, is further complicated by the fact that there is not always a network configuration which allows for a sufficiently sensitive determination of insulation faults in the faulty subnetwork.
Apart from these difficulties in locating insulation faults, there is the problem in continuous insulation monitoring, too, that, according to the current state of the art, it is not possible to have the IT system actively monitored by more than one insulation monitoring device at a time. The active measuring systems of two or more active insulation monitoring devices in an IT system can influence one another in such a manner that the monitoring task is not reliably ensured. Since the conductance of the parallel circuit of all complex-valued insulation resistances in the entire system is always monitored, i.e. since the insulation monitoring device sees all network leakage capacitances present in the IT system, the measuring system of the insulation monitoring device has to be configured in such a manner that it can handle the interferences generated by the subsystems. In some applications, the selective monitoring of IT subsystems is required to be of such a design that an insulation fault in one IT subsystem is to lead to the quick shutdown of the affected subsystem without influencing other IT subsystems. So far, this requirement cannot be met by insulation monitoring systems according to the state of the art.
In cases where selective insulation monitoring is needed and in the required reaction to a critical second fault in the IT system, efforts are made to realize expedient solutions by using direction-selective differential current measuring technology and in a simple manner also by means of overcurrent triggers. These solutions, however, can be used reliably in IT systems only under certain configurations. In direction-selective differential current measuring technology, the ratio of the network leakage capacitances upstream of the summation current transformer and of the network leakage capacitances downstream of the summation current transformer is crucial for the direction-selective measurement to function reliably.